1. Field of the Invention
The present invention relates to a radar apparatus and a sonar apparatus which output detected image data based on a detected signal to a display device, and apparatuses similar thereto.
2. Description of the Related Art
Conventionally, image processing which executes a correlation process with respect to images obtained by several past scanning operations is performed so as to suppress an unnecessary video, such as sea surface reflection or the like. This process is referred to as “scan correlation process”.
A radar apparatus which is carried on a mobile object, such as a ship or the like, needs to stabilize a change in course and a change in position before performing a correlation process, and therefore, receives compass data for detecting a ship's head, and velocity or latitude and longitude data for detecting a change in position, and uses these data values to generate an address in an image memory for the correlation process. Specifically, a sweep direction (θ) is assumed to be equal to a value (θ=θr+θc) obtained by adding the ship's head (θc) with an antenna's relative direction (θr) where the ship's head is used as a reference, and a sweep start point position is changed corresponding to a movement amount of the mobile object, thereby generating an address in the correlation process image memory. In other words, the address start point for the scan correlation process is a true motion coordinate point. Although this should be described in the following description, this complicates the description of the present invention and is also known, and will not herein described in detail.
FIG. 12 is a block diagram illustrating a configuration of a conventional general radar apparatus.
The conventional radar apparatus includes a radar antenna 101, a receiver 102, an AD converter 103, a sweep memory 104, a drawing start point generator 105, a drawing address generator 106, a display image memory 107, and a display device 108.
The radar antenna 101 transmits a transmission signal composed of pulsed radio waves in predetermined transmission cycles while rotating on a horizontal plane in predetermined cycles. At the same time, the radar antenna 101 receives radio waves reflected from a target present within a detection area. The receiver 102 detects and amplifies the received signal and outputs a detected signal. The AD converter 103 subjects the detected signal to AD conversion to generate detected data represented by a polar coordinate system. The sweep memory 104 stores one sweep of detected data based on a distance in real time. The sweep memory 104 outputs one sweep of detected data thus stored to the display image memory 107 by the time when detected data obtained by the next transmission is received. The drawing start point generator 105 sets a draw start point address (Xs, Ys) for detected image data which is to be drawn in the display image memory 107, and outputs the draw start point address (Xs, Ys) to the drawing address generator 106.
The drawing address generator 106 generates addresses which designate pixels in the display image memory 107 arranged in a corresponding rectangular coordinate system, based on an antenna angle 0 where a predetermined direction (e.g., a ship's head) is used as a reference, and a read position r in the sweep memory 104 stored based on a distance, directing outward from a sweep start point as a start address. Specifically, the drawing address generator 106 is composed of hardware which realizes the following expressions.X=Xs +r−sin θY=Ys +r−cos θwhere X and Y are an address which designates a pixel in the display image memory 107, Xs and Ys are the above-described draw start point address, r is a distance from the center of sweep, and θ is a sweep angle.
The display image memory 107 has a capacity which can store detected image data corresponding to one revolution of the antenna. The data is read out from the display image memory 107 with high speed in synchronization with raster scanning of the display device 108 performed by a display controller (not shown). The detected image data thus read is assigned with an intensity or color corresponding thereto, and is output as display image data to the display device 108. The display device 108 displays an image on a screen based on the received display image data.
In the case of a target detecting system using such a radar apparatus, if a structure which reflects radio waves is present near the antenna, the structure obstructs detection of a target behind the structure as viewed from the antenna. FIG. 13 is a schematic diagram illustrating an area which cannot be detected due to the positional relationship between an antenna installed position and an obstruction. For example, as illustrated in FIG. 13, when containers 201 are stacked on the deck of a ship body 200, and the radar antenna 101 is installed above a wheelhouse positioned at the stern of the ship body 200, radio waves transmitted from the radar antenna 101 are interrupted by the container 201, so that a target present in a predetermined area 210 cannot be detected.
The radar has a more satisfactory long-range detection capability as the radar antenna position is made higher. Conversely, the higher radar antenna position leads to a wider range of sea surface reflection video captured. On the other hand, the short-range detection capability of the radar depends on the vertical beam width of the radar, and is more satisfactory as the radar antenna position is made lower.
Therefore, a plurality of radar antennas are provided, videos received by the radar antennas are displayed on respective display devices, and a display device suitable for an observation purpose is selected among these display devices and is observed. FIG. 14 is a schematic diagram illustrating a detectable area when two radar antennas 101 and 111 are provided. In the example of FIG. 14, by installing the radar antenna 111 at the bow of the ship body 200, an area corresponding to the area 210 of FIG. 13 is detected by the radar antenna 111, thereby removing the non-detectable area. In addition, the radar antenna 101 is installed at a high position, and the radar antenna 111 is installed at a low position, thereby making it possible to achieve both long-range and short-range detection capabilities.
As illustrated in FIG. 15, there is an apparatus in which the radar antennas 101 and 111 are connected via a switch module 303 to two display devices 108 and 118. FIG. 15 is a schematic diagram illustrating a configuration of a radar apparatus which switches and displays a plurality of detected images using the switch module 303.
JP No. H4-238285A discloses another conventional radar apparatus which includes a plurality of radar antennas, and outputs detected data obtained from these radar antennas to a single display device.
However, in the case of the conventional configuration comprising a plurality of radar antennas of FIG. 14, a display device is typically provided for each antenna. Therefore, the operator needs to observe targets in detection areas while viewing the display devices separately, depending on the situation.
In the configuration of FIG. 15, the switch module 303 connects receivers 301 and 302 to the respective display devices 108 and 118 in one-to-one correspondence. Therefore, each display device can display a detected image from only either of the radar antennas.
In the radar apparatus described in JP No. H04-238285A, the same (single) draw center is used for detected data obtained from a plurality of radar antennas. However, when images obtained from a plurality of radar antennas installed at separate places are combined and displayed on a single display device, and the same position is used as a draw center, an image of the same target is displayed at a plurality of different positions on the display device, resulting in an incorrect image. In such a case, the draw centers of the received images need to be placed at different positions corresponding to the respective antenna installed positions. An error in the above-described display position increases with an increase in a distance with which the installed positions of the radar antennas are separated from each other, or with a decrease in a detection range within which observation is performed.